Constant deviation scanning apparatus

ABSTRACT

A constant deviation beam scanning apparatus having a rotatable beam deflector with at least one reflective surface thereof lying in a plane that intersects the axis of rotation at an acute angle. One embodiment utilizes a rotatable, truncated, n-sided pyramidal mirror with at least one planar reflective scanning segment located on one of the n-sides of the pyramidal mirror. The pyramidal mirror&#39;s truncation surface lies in a plane normal to the mirror&#39;s rotation axis and contains at least one planar reflective segment. A constant deviation reflector having two reflective surfaces is positioned so that a collimated beam of light reflected by the truncation surface reflective segment is reflected by one of the constant deviation reflective surfaces to the other reflective surface and then to the at least one planar reflective scanning segment of the pyramidal mirror.

This is a divisional of co-pending application Ser. No. 035,974, filedon Apr. 8, 1987, now U.S. Pat. No. 4,793,672.

BACKGROUND OF THE INVENTION

The present invention relates to optical beam scanning apparatus and,more particularly, to a wobble compensated beam scanning apparatushaving a rotatable beam deflector with at least one reflective surfacethat lies in a plane which intersects the axis of rotation at an acuteangle.

Optical beam scanners having a rotating, pyramidal mirror for scanning awriting or reading beam across a media-containing image/object plane arewell known in the art. Representative examples of such pyramidal mirrorscanning systems are found in a variety of the now ubiquitous "laserprinters" and "laser scanners". The pyramidal mirrors used in suchprinters and scanners can be divided into two major categories: one inwhich the planes of the reflecting surfaces are parallel to therotational axis of the pyramidal mirror; and, two, those in which theplanes of the reflecting surfaces of the pyramidal mirror intersect themirrors' rotation axis at an acute angle.

The quality of the output media from the printers and scanners dependsin a large measure upon the positional accuracy of the scanning beam. Inorder to maintain the desired degree of beam placement accuracy,relative tight positional requirements are imposed upon both thereflecting surfaces of the pyramidal mirror and the rotation axis of themirror.

The rotation axis of the pyramidal mirrors in both categories mustremain "true" with respect to the optical system. If the rotation axisdeparts from the "true" rotation axis, a cross-scan error is introducedin the optical system. The departure of the mirrors' rotation axis fromthe "true" rotation axis is caused by "wobble" in the scanningapparatus. The wobble can be either random or fixed or a combinationthereof. Random wobble is introduced by loose spin motor bearings,non-spherical ball bearings and the like, while the fixed wobble isintroduced when the rotation axis of the pyramidal mirror is misalignedwith respect to the "true" rotation axis. The wobble induced cross-scandeviation must be compensated for in order to achieve a high degree ofaddressability in the image/object plane.

One embodiment of the present invention is directed to wobblecompensated beam scanners employing pyramidal mirrors of the secondcategory in which the planes of the reflective scanning segmentsintersect the axis of rotation at an acute angle.

The use of a rotating, pyramidal scanner is old in the art. See U.S.Pat. No. 3,487,224, issued Dec. 30, 1969 for a "Scanner Which Utilizes APair Of Time-Shared Apertures". Truncation of the rotating pyramidalreflective scanning element is shown in the following U.S. Pat. No.:3,866,038, issued Feb. 11, 1975 for "Apparatus For Measuring SurfaceFlatness"; U.S. Pat. No. 4,268,110, issued May 19, 1981 for "Facet AngleCorrector For Multi-Faceted Optical Scanner"; U.S. Pat. No. 4,312,590issued Jan. 26, 1982 for "Optical Scanner And System For Laser BeamExposure Of Photo Surfaces"; and, U.S. Pat. No. 4,323,307 issued Apr. 6,1982 for "Light Beam Scanning Apparatus".

The 4,312,590 and 4,323,307 patents disclose a wobble compensatedscanner utilizing a rotating, truncated pyramidal reflector with tworoof mirrors. The truncated portion of the pyramidal reflector is notemployed either as a reflecting surface or as an active element in theoptical system. In each patent, wobble compensation is achieved onlythrough multiple reflections between the rotating, truncated pyramidalmirror sides and the roof mirrors.

It is accordingly a general object of the present invention to providean improved wobble compensating beam scanner.

It is a specific object of the invention to provide a wobblecompensating beam scanner having at least one reflective surface thatlies in a plane that intersects the axis of rotation at an acute angle.

It is another object of the invention to provide a beam scannerutilizing a rotating, truncated pyramidal mirror with the truncationsurface acting as a wobble compensating optical element in the scanner.

It is still another object of the present invention to provide a wobblecompensated beam scanning apparatus having a compact form factor for usein laser beam printers and scanners.

It is a feature of the invention that multiple scans of the read/writemedia can be achieved during a single rotation of the wobble compensatedrotating, truncated pyramidal mirror.

It is another feature of the invention that the reflectingcharacteristics of the reflecting surfaces of the pyramidal mirror canbe varied to accommodate intensity variations in the laser beam lightsource.

SUMMARY OF THE INVENTION

The beam scanning apparatus of the present invention employs a rotating,truncated pyramidal mirror having n-sides of which at least one sidecontains a planar reflective scanning segment. The truncation surface ofthe mirror contains a planar reflective segment. A constant deviationreflector comprising two reflective surfaces is positioned so that acollimated light beam reflected by the truncation surface planarreflective segment is reflected by one of the reflective surfaces of theconstant deviation reflector to the other surface thereof and then tothe planar reflective scanning segment located on one of the n-sides ofthe rotatable, truncated pyramidal mirror. A spin motor is used torotate the mirror. If the axis of rotation departs from the desired"true" rotation axis, the wobble, either random and/or fixed producedthereby, is compensated by the action of the combination of thetruncation surface planar reflective segment located on the truncationsurface and the constant deviation reflector. An F-theta scan lensfocuses the beam onto an image/object plane.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention described above will best beunderstood from a detailed description of a preferred embodimentthereof, selected for purposes of illustration, and shown in theaccompanying drawings in which:

FIG. 1 is a diagrammatic view in partial block form showing an imagingsystem incorporating the wobble compensated beam scanning apparatus;

FIG. 2 is a view in side elevation and partial section showing inenlarged form the beam scanning apparatus illustrated in FIG. 1;

FIG. 3 is a plan view of the rotating, truncated pyramidal mirror shownin FIG. 2;

FIG. 4 is an enlarged diagrammatic view in side elevation illustratingthe wobble compensating action of the beam scanning apparatus;

FIG. 5 is a view similar to that of FIG. 2 depicting the use of a filterwheel in conjunction with the rotating, truncated pyramidal mirror;

FIG. 6 is a plan view showing the filter wheel illustrated in FIG. 5;

FIG. 7 is a view in side elevation showing the use of a right prism asthe rotating beam deflector;

FIG. 8 is a plan view of the rotating prism depicted in FIG. 7;

FIG. 9 is a view in side elevation showing the use of a right angleprism in which rotation of the prism produces a 360° scan in a planeperpendicular to the axis of rotation; and,

FIG. 10 is a plan view of the prism of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, and particularly to FIG. 1 thereof, thereis shown a scanning system indicated generally by the reference numeral10 which comprises three major components: a beam scanner indicatedgenerally by the reference numeral 12, write/read folding opticsindicated generally by the reference numeral 14 and a media stationindicated generally by the reference numeral 16. Each of these majorcomponents will be discussed below in greater detail.

The beam scanner 12 comprises a rotatable, truncated, pyramidal mirror18 driven by a spin motor 20. The rotatable, truncated, pyramidal mirrorhas at least one planar reflective scanning segment 22, the plane ofwhich intersects the rotation axis of the truncated pyramidal mirror 18at an acute angle. In addition, the rotating truncated, pyramidal mirror18 has a wobble compensating planar reflective segment 24 located in theplane of the truncation surface of the mirror 18.

A frequency modulated laser diode 26 generates a write beam 28.Similarly, a reference beam laser diode 30 generates a reference beam32. The write beam 28 and reference beam 32 are directed by means ofbeam splitter 34 to the truncation surface planar reflective segment 24from which they are reflected to a constant deviation stationary mirrorpair indicated generally by the reference numeral 36 and comprisingmirrors 36a and 36b. After reflection from mirrors 36a and 36b, the twobeams impinge upon the planar reflective scanning segment 22 of therotating truncated pyramidal mirror 18 and, after reflection from thissurface, the beams pass through a focusing scan lens 38 of the f-thetatype.

The write and read beams 28 and 32, respectively, exiting from scan lens38 are directed through a periscope mirror pair, indicated generally bythe reference numeral 40, and comprising mirrors 40a and 40b. At thispoint, the reference beam 32 is directed to fold mirror 42 and thenthrough a reference beam grating encoder 44 to a curved mirror 46. Thereference beam 32 is reflected from the curved mirror 46 to a photodiode 48. The function of the photo diode 48 will be discussed below inconnection with the modulation of the frequency modulated write beamlaser diode 26.

After reflection from mirror 40a, the write beam 28 impinges upon amedia 50 at an image/object plane 52. The scan direction of the writebeam 28 is normal to the plane of FIG. 1, i.e., the beam is scanned inand out of the drawing. A transport system for media 50 isrepresentatively shown by means of a drive roll 54.

It has been mentioned previously that the write beam laser diode 26 isfrequency modulated. The modulating signal represents stored data 56that is fed to the write beam laser diode 26 through a data out controlcircuit 58. The data out circuit 58 receives a control signal input fromshaft encoder 60 and from pixel clock 62. In order to time themodulation of laser diode 26 with respect to the position of therotating at least one planar reflective segment 22, the shaft encoder 60produces an enable/disable signal for data out circuit 58 as a functionof the angular position of the rotating pyramidal mirror 18 with respectto a fiducial (not shown).

The output from photo diode 48 is used to set and update pixel clock 62to control the rate of data flow from data source 56 through the dataout circuit 58 to the write beam laser diode 26. The encoder 44, photodiode 48 and pixel clock 62 thus provide a velocity compensation forchanges in the velocity of the write beam 28.

Referring to FIGS. 2 and 3, the beam scanner 12 shown in FIG. 1 isdepicted in enlarged form and greater detail in FIG. 2 and the rotatingtruncated pyramidal mirror 18 of FIG. 1 is shown in enlarged form andgreater detail in FIG. 3. The optical and mechanical components of thebeam scanner 12 are mounted in and with respect to a housing 64. As seenin FIG. 2, the rotating, truncated pyramidal mirror 18 is mounted on asuitable mounting plate 66 that in turn is secured to shaft 68 of thespin motor 20. A light trap 70 is provided to trap light passing throughwedge beam splitter 34 from the reference beam laser diode 30.

It can be seen in both FIGS. 1 and 2 that the write beam 28 from writebeam laser diode 26 passes in transmission through the beam splittingwedge 34 while the reference beam 32 is reflected therefrom. Since thewrite beam passes through the beam splitter 34 in transmission,vibration will not produce an error in the write beam.

Referring to FIG. 3, the rotating, truncated pyramidal mirror 18 has atleast one reflective scanning facet, e.g., 22a. If desired a pluralityof reflective scanning facets can be located on the truncated, pyramidalmirror as indicated by the reference numerals 22b, 22c and 22d.Depending upon the number of reflective scanning segments, acorresponding number of scanning lines can be produced at theimage/object plane 52 for one rotation of the truncated, pyramidalmirror 18.

If more than one reflective scanning segment 22 is employed on therotating, truncated pyramidal mirror 18, the reflective scanningsegments can have the same or differing degrees of reflectivity. Byemploying differing degrees of reflectivity for the reflective scanningsegments 22, it is possible to accommodate variations in the intensityof the writing beam laser diode 26 if the diode is replaced. Since eachreflecting scanning segment 22 can have a different degree ofreflectivity, this capability, in conjunction with the timing providedby a shaft encoder 60, the output from photo diode 48 and pixel clock 62can be used to control the intensity of the write beam 28 at theimage/object plane 52.

The wobble compensation provided by the rotating truncated pyramidalmirror 18 is illustrated in enlarged form in FIG. 4. The solid arrowedlines in FIG. 4 represent one position of the write beam 28 with respectto the wobble compensating reflective facet 24 and the reflectivescanning facet 22. If the rotating, truncated pyramidal mirror 18"wobbles" to a position shown by the dashed lines in FIG. 4, the writebeam 28 will be reflected from the wobble compensating reflective facet24 as shown by the dashed arrowed lines in FIG. 4. It can be seen fromFIG. 4 that both of the write beams identified as 28' and 28" reflectedfrom the reflective scanning segment 22 are parallel to each other and,therefore, will be focused by the scanning lens 38 at the same point onimage/object plane 52.

Referring to FIGS. 5 and 6, there is shown in side view (FIG. 5) andplan view (FIG. 6) a filter wheel 72 that is secured to the truncatedsurface 24 of the pyramidal mirror 18 by means of fasteners 74. Thefilter wheel 72 contains one or more filter segments represented by thereference numerals 72a-72d in FIG. 6. The filter element or elements72a-72d can be used to perform a variety of functions. For example, ifthe filter elements comprise neutral density filters of varyingstrength, the filter wheel 72 can be used to compensate for variationsin the intensity of the write beam laser diode 26. If a polychromaticlight source is used to generate the write beam 28, the filter wheel 72can include color filters to permit additive or subtractive color beamillumination of a photosensitive medium 50 at the image/object plane 52.If the beam scanning system 10 is used to read a media 50 at theimage/object plane 52, the color filter wheel 72 will provide theappropriate color scanning images to a photosensitive receiver (notshown) which replaces the write beam laser diode 26.

Looking now at FIGS. 7 through 10, there is shown further embodiments ofa constant deviation scanner utilizing a constant deviation prismindicated generally by the reference numeral 76 in FIGS. 7 and 8 and bythe reference numeral 78 in FIGS. 9 and 10.

The prism 76 comprises a right angle prism in which the hypotenuse 80 issilvered in order to obtain internal reflection. A collimated beam ofincident radiation 82 illuminates prism face 84 and passes through theface and is reflected from the silvered surface of hypotenuse 80 andback to face 84 from which it is totally internally reflected. Given thesymmetry of prism 76, if the prism is rotated about an axis 86 that isperpendicular to the hypotenuse 80 and bisects the 90 degree apex of theprism, the exiting beam 88 will scan in a horizontal plane (if the axisof rotation 86 is viewed as vertical) twice per revolution of the prism.

Referring to the FIG. 8 plan view of prism 76 shown in FIG. 7, it can beseen that the apparent center line of rotation 90 of the scanned exitingbeam 88' and 88" appears to coincide with the center line 82a of theincident beam 82 and not the mechanical axis of rotation 86.

If the positions of the incident and exiting beams 82 and 88,respectively, are exchanged and a new axis of rotation 92 is madeparallel to the center line 82a of the new incident beam as shown inFIGS. 9 and 10, rotation of the prism about axis 92 will cause theexiting beam 88' and 88" to scan continuously through 360 degrees in aplane perpendicular to the axis of rotation 92. The center of opticalrotation 94 is again coincident with the center line of 82a of theincident beam 82. Although only one useful scan is generated perrotation of the prism 78 in the configuration shown in FIGS. 9 and 10,it is possible to mount a scanning lens much closer to the opticalcenter of rotation 94 thereby significantly decreasing both the size andcost of the focusing scan lens.

Having described in detail embodiments of my invention, it will now beobvious to those skilled in the art that numerous modifications can bemade therein without departing from the scope of the invention asdefined in the following claims.

What I claim is:
 1. A beam scanning apparatus comprising:a rotatableright angle prism having first and second faces and a hypotenuse side,said hypotenuse side having an inwardly facing reflective segment; meansfor rotating said right angle prism about an axis that is normal to thehypotenuse side and bisects the 90° apex of the prism; and, means forimpinging a beam of collimated light on the first face of said prism sothat the light beam passes through the first face, is reflected by theinwardly facing reflective segment of the hypotenuse side and thentotally inwardly reflected by said first face before exiting the prismthrough the second face.
 2. A beam scanning apparatus comprising:arotatable right angle prism having first and second faces and ahypotenuse side, said hypotenuse side having an inwardly facingreflective segment; means for impinging a beam of collimated light onthe first face of said prism so that the light beam passes through thefirst face, is totally inwardly reflected by the second face of theprism and then by the inwardly facing reflective segment of thehypotenuse side before exiting the prism through said second face; and,means for rotating said right angle prism about an axis parallel to acollimated light beam that impinges on the first face of said prism.